The present invention relates generally to the calibration of measurement devices and more particularly pertains to the calibration of measurement systems wherein remote sensors optically interact with analytical instruments.
Measuring devices often comprise multicomponent systems wherein a remote sensor or probe component generates a signal in response to a certain condition and a processing or analyzing instrument is employed to convert such signal into meaningful data. Both the sensor component as well as the processing component are typically subject to variation in that the actual signal generated by a sensor in response to a given condition may vary from sensor to sensor and the output generated by the instrument in response to a given signal as received from a sensor may vary from instrument to instrument. It is therefore necessary to calibrate the sensor component, the instrument component or both such that accurate results are obtained in response to given conditions. Calibration efforts are considerably more complex in systems wherein any of a plurality of probes are intended to interact with any of a plurality of instruments. Calibration efforts are further complicated in systems wherein the raw signal generated by the probe is at least partially dependant upon instrument input. Additional problems are inherent in systems wherein electronic and optical componentry is combined.
Certain invasive optical blood gas analyzers are examples of measurement systems subject to all of the above set forth complexities relating to calibration. Such systems present a selected fluorescing medium to blood flow, irradiate the medium to induce fluorescence and compare the excitation radiation's intensity with the intensity of the resulting fluorescence. The medium is selected such that its rate of fluorescence is quenched by the presence of a certain gas to render the resulting intensity ratio a function of the concentration of such gas. A probe employing the described medium, when introduced into a patient's vasculature, can therefore provide real time indications of the partial pressures of certain gasses within the patient's blood supply. Because such probes cannot be reused, the system must be designed to render their disposability economically feasible.
The type of invasive optical blood gas analyzing system especially difficult to calibrate is a system wherein the excitation signal is generated within the analyzing instrument, conducted to the probe via an optic fiber, and fluorescence, emitted by the probe, is returned to the instrument via the optic fiber for analysis. By retaining a substantial portion of the optical hardware within the instrument, the cost of the probe is substantially reduced but considerable calibration problems are introduced as a direct result of such a separation of the optics. Variations inherent in the probe include the sensitivity of the particular deposit of fluorescing medium employed therein and the transmission qualities of the optical conduit and optical coupler. Variations inherent in the instrument include the output of the radiation source, the sensitivities of the sensors measuring the outgoing and incoming radiation intensities as well as the transmission qualities of the optical conduits and couplers. Simply calibrating the probe will not compensate for variation in the instrument and vice versa. In order for the system to produce accurate results, all these sources of variation must be compensated for with respect to each individual instrument and probe combination.
While the calibration of each probe and instrument combination just prior to use would ensure accurate results, such calibration efforts are not always practical or even possible in the environment where and under the conditions which such blood gas analyzers are typically put to use. It is often desirable to be able to transfer a particular probe from one instrument to another without the need to recalibrate the new probe and instrument combination upon transfer. Such situations arise when transferring a patient from an operating room to a recovery area where the movement of the analytical instrument is impractical. It is most desirable to be able to leave the probe in position within the patient's vasculature, disconnect the probe from the instrument located in the operating room, transfer the patient into any of a number of recovery areas and immediately reconnect the probe to an instrument located there. Removing the first probe and inserting a new probe calibrated to the second instrument is contraindicated due to the increased probability of infection and the additional effort involved. A number of calibration techniques have heretofore been suggested in an effort to overcome this "transportability" problem inherent in this type of analytical equipment, but each suffers from substantial shortcomings as set forth in more detail below.
It has been suggested that upon arrival in the recovery area, a blood sample would be drawn for analysis and that the second instrument's output would then merely be adjusted to conform to the lab results. This however assumes that the second instrument's calibration is merely in need of an offset adjustment and ignores any slope changes that may in fact be necessary. Moreover, the patient's blood gasses may be subject to substantial fluctuation during the time elapsed between the time when the blood sample was drawn and the time when the instrument is actually recalibrated. Such errors would most likely occur in the case of an unstable patient while it is precisely the unstable patient that is most dependent upon accurate information.
An alternative approach has been proposed wherein a dual sensor probe component is used in conjunction with appropriately modified analyzing instrumentation. One of the sensors is intended for introduction into the patient's vasculature while the second sensor remains available for calibration at all times. This approach, however, requires the two sensors to be identically responsive to the presence of the gasses being tested for, which may introduce considerable if not insurmountable manufacturing problems. Moreover, such modification would add considerable cost to that component of the system which is intended to be disposed of after every use. Adaptation of the analyzing instrument to accommodate an additional sensor and to process information generated thereby would further add considerable cost to the system. Finally, although such approach allows a probe to remain within a patient and provide accurate information when interconnected to a succession of instruments, a skilled labor-intensive calibration effort is nonetheless required with each transfer.
Alternatively, it has been suggested to integrate the optical components of the instrument in a portable optics module that remains interconnected to the probe residing within the patient at all times. Upon transfer, the optics module is disengaged from the analyzing instrument and transported to the recovery area where it is simply plugged into the second instrument. Incorporation of such a feature would, however, add cost to the instrumentation, as this approach does require that extra equipment be transported with the patient and logistical problems are posed by the necessity of keeping track of numerous such modules throughout a typical medical facility.
Another alternative approach involves the use of a universal standard to which all of the instruments in use would be calibrated such that a given signal received from any probe would yield the same value on every instrument. Since instrument performance is subject to drift and degradation, calibration of the instruments would have to be performed on a periodic basis and cannot simply be permanently accomplished at the time of manufacture. Return of the instruments to a central facility for periodic recalibration would be an impracticable alternative, so this approach would require the development of calibration standards which could engage the instruments in the field. Such calibration standards would have to be sufficiently stable so as to be transportable all over the world, yet capable of exactly representing actual probes in all optical respects. The development and production of such a universal standard is a formidable undertaking. The necessity for acquiring and maintaining such standards would add cost to the system.
The prior art is devoid of a practical solution for maintaining a plurality of analyzing instruments of the type described in calibration. An approach is called for that allows a probe to be transferred from instrument to instrument without the need to undertake any recalibration efforts and that achieves such function without a substantial increase in cost and complexity.